The main objective of silkworm breeding programs is collecting the appropriate genes in the populations to improve productive performance lines (Jayaswal et al., 2000). The general aim of selection in animal breeding is to obtain new generation of animals, which under future economic conditions of production system are more efficient than the present generation. In selection index programmes, top animals are selected based on their multiple-trait genetic-economic value named genetic merit. In this method, genetic trend of important traits is directly based on their weights in economic efficiency of production system to acquire maximum improvement in economic merit (Shadparvar et al., 2005). The objective of this study was to study changes in the genetic merit of 6 silkworm lines over 8 generations.